1. Field of the Invention
This invention relates to a precision machine tool and to a machining apparatus that performs high precision machining.
2. Description of the Related Art
To create a die for molding a light guide plate of the type used for backlighting in a liquid crystal display, tens to hundreds of thousands of minute dimples (hemispherical hollows) need to be machined on the surface of a workpiece. The dimples have a diameter of tens to hundreds of micrometers and are conventionally created one by one by cutting with a machine tool. To perform the cutting, it is necessary to repeat a procedure that includes high speed driving of a small-diameter ball end mill, horizontal positioning of the X and Y axes of the machine tool, and cutting in the Z-axis direction to create a hemispherical dimple. Sometimes the machining is done by a laser machine, in which case some tens of dimples can be machined per second.
A patent application for an invention related to a machining apparatus that performs machining in accordance with a light and dark pattern was filed on Jul. 6, 2005 (Japanese patent application No. 2005-203338). This machining apparatus has a linear driving means that moves a movable part linearly, a light and dark patterned member having a light and dark pattern that indicates machining information, and a sensor that moves relative to the light and dark pattern in synchronization with the movement of the movable part and outputs a light and dark pattern scanning signal. The movable part has a tool and a cutting means that changes the cutting depth of the tool according to the signal from the sensor. As the movable part and the sensor move, the sensor scans the light and dark pattern and the cutting means varies the cutting depth according to the signal received from the sensor, so that machining is performed in accordance with the light and dark pattern.
No publicly available documentation of the use of a light and dark pattern in a method of performing intermittent machining such as the machining of a die for a light guide plate could be found.
Machining hundreds of thousands of dimples on a light guide plate one by one by a cutting process is inefficient because it takes too much time. A problem in high speed machining with a small-diameter ball end mill is that the ball end mill is easily broken. If it were possible to perform a periodic cutting process by advancing a non-rotary tool such as a diamond cutting tool continuously in the horizontal direction, the machining time could be reduced by eliminating the dwell time for each cut.
Ordinary machine tools are limited, however, to machining at most a few dimples per second due to the limited speed of the numerical controller or the limited response speed of the machine tool. When a laser machine is used, the machining speed becomes higher, but this machining method uses a melting process, so the machining profile is less precise than in a cutting process. In both cutting and laser machining, if a position for each dimple is specified in a program, the size of the program becomes very large, including hundreds of thousands of lines, even if only one line is required for each dimple.